So the main quest says to get the attention of the thieves guild in Vastares. I read posts about going to another city for the Thieve’s Guild quest. Within the game how do I get info in Vastares that tells me what to do next?

The answers to your question are all there in that guide. The Main Mission does say, "Your main, but secret mission on the island of Freymore is to find out more about the missing Governor and about an ancient prophecy called The Returning. You may get more information in the city of Vastares, where you need to attract the attention of the Thieves' Guild. Governor Hume's castle can be found in the city of Mithria."

So keep all that in mind and talk to the Mysterious Man. He's the key to solving the Main Mission. He'll have a lot of tasks for you. So as soon as you can, get the quest, "Join the Thieves' guild in Vastares. To attract their attention, you need to infiltrate one of their transports (in the swamp area, around the Lake of Freymore), then knock on the inn's door at the port of Vastares. Once you're inside the Guild's lair, break into the private parts, grab the mighty magical items against the Leader of the Mage Council, and bring them back to the Mysterious Man in Phryn."

You can "slide" to Mithria by hugging the coastline and moving very fast. Don't engage in battles. Just move fast. Save when you can in a different slot. I've done it so many times from Matras to Mithria with a new player but don't know how better to describe it.